[Chris Hornbeck]

On Fri, 11 Jan 2008 21:49:20 -0500, Randy Yates
wrote:

In the past we were told that negative feedback in an amplifier (power
amplifier) was bad. I believe the old charge was that it produced
excessive "transient intermodulation distortion."

Can someone please explain, using as much engineering-speak as necessary
(i.e., don't sugar-coat it - assume an audience of electrical engineers)
what this was all about?

The classic 1970's description went: an amplifier is modelled
as an input transconductance stage driving a capacitor, the
value determined by what's necessary for single dominant pole
stability, a big-ass gain stage, and followers.

The limiting condition is when all the input transconductance
stage's current is used in charging and discharging the
capacitor, slewing. But the reactive loadline causes distortion
effects at lower levels, called, tada, TIM, or various other names.
"SID" (slewing induced distortion) was also used.

This hasn't magically disappeared as an amplifier design issue,
but awareness of the importance of degenerating the transconductance
of the input stage, and much better devices, have helped move
it off the hot seat.

Although it was often framed within the context of an issue
of feedback, it never really was. It was really an issue of
input stage transconductance in a single dominant pole
feedback amplifier.

Trimming 2/3 of my response, because I can go on forever,
sorry,

Much thanks, as always,

Chris Hornbeck

"There's little that's impossible, but it becomes more complicated if
you move between different systems." - Mike Rivers, in another context